Small pimples on the arms are often a result of clogged pores, oil production, dead skin cells and bacteria, according to Mayo Clinic. When glands secrete oil that lubricates skin and hair, hair follicles become plugged, which contributes to production of small pimples.
Pimples become visible when follicles become infected or inflamed, according to Mayo Clinic. Little bumps that appear to be pimples on the arms may also be caused by keratosis pilaris, which is a skin condition caused by a build up of protein keratin that plugs hair follicles, according to WebMD. Prescription medication and ointments from a dermatologist can treat this condition.